Thermosyphon Solar
Air Space Heater (with 1 year payback)

These articles describe the simple and inexpensive
passive solar collector that I use to heat my shop/barn. The total
cost for the 150 sqft collector is about $350, and all the materials are
available at your local hardware/lumber store. No fans or electronics --
its driven by natural thermosyphoning. The design can be adapted
to suit a wide variety of spaces, including a living space. At our current
propane prices ($2.00 per gallon), this project has a one year payback.

I would suggest reading
both the Mother Earth and Home Power articles (below), as they each have some unique
material that will be helpful if you are building the collector. Also
review the 2012 updates section for some potential refinements.

2012 Updates and Changes

The collector was built in the winter
of 2003-2004 and has been faithfully pumping out heat ever since. This is
an update on how things have gone with it, some recent performance testing, and
a new "commercial" version of the collector:

Note: I noticed that the design guidelines for thermosyphon collectors did not
make it into the document above, so here they are:

Design Guidelines for Thermosyphon Collectors

The buoyancy forces that drive the air flow through a Thermosyphon collector are
weak.
The collector must be designed to minimize flow resistance. High flow resistance
will
result in higher collector temperatures, higher losses, and lower efficiency.
For low flow
resistance, flow passages must be large, and the absorber must transfer heat to
the air
efficiently and with little flow resistance.

The depth of the collector should
be about 1/15th of the height (more is better).

The top vent and bottom vent areas
should each be at least 50% of the collector cross
sectional area (more is better).

The air flow path through the
collector should be as shown in the “How It Works”
sketch. This path keeps the cooler air near the glazing to reduce losses to
the outside,
and also provides good heat transfer from the absorber to the air as it passes
through
the absorber.

The absorber must 1) have low air
flow resistance, 2) absorb a large fraction of the
incident solar energy, and 3) transfer heat to the air well. The window screen
absorber
seems to do this well, but other materials may also work.

Streamlining the entrance and exit
vent areas may also help reduce airflow resistance
and increase collector efficiency.

Changes that improve collector flow
path will normally result in a decrease collector
output temperature along with a more than compensating increase in flow rate.
For best
efficiency you want a lot of air flow with a modest temperature rise.

Front view of collector with main parts labeled (picture
from Home Power Article)